This invention relates to a high strength hot rolled steel sheet having an excellent stretch flangeability together with a high tensile strength of the level, for example, at least 980 MPa, and its production method. This hot rolled steel sheet is useful as a material for use in reinforcing, for example, a member, a bumper, or a pillar of an automobile.
Development is underway to provide hot rolled steel sheets having a high strength as measured by tensile strength of the level as high as 980 MPa or higher simultaneously with an improved stretch flangeability for realizing high forming workability. Means that have been used in realizing the high stretch flangeability include use of a single phase structure such as ferrite or bainitic ferrite for the metal structure, and use of a complex structure of ferrite and bainite in which difference in strength between the two phases is suppressed to the least level. Also known are techniques using precipitation strengthening by microalloying that have been used to realize the high strength while maintaining such structure. For example, Japanese Patent Application Laid-Open Nos. 2000-109951, 2002-180189, 2003-89848, 2004-204326, and 2004-285420 disclose attempts to attain the high strength by adding Ti, which is an element used for precipitation strengthening.
In the meanwhile, Japanese Patent Application Laid-Open No. 2003-105446 discloses a technique for realizing the high strength and the workability which does not use the precipitation strengthening of the alloying element. In this technique, such properties are realized by forming a metal structure predominantly including martensite by rapidly quenching the hot rolled steel sheet immediately after the hot rolling.
These techniques, however, suffer from various problems. In the techniques using precipitation strengthening by the addition of a microalloy as mentioned above, the slab should be reheated to a high temperature before the hot rolling due to the addition of Ti and the like at a high content, and such use of high temperature is disadvantageous in terms of heat energy. Improvement of the strength by such use of precipitation strengthening by the addition of Ti and the like is also associated with the problem of significant increase of yield ratio, which results in the poor form stability after the pressing.